1. Field of the Invention
The invention relates to a lithography simulation technique used for manufacturing a semiconductor device, and more particularly to a simulation method for simulating light intensity on a substrate in a lithography process. Further, it relates to a program for carrying out the simulation method in a computer.
2. Description of the Related Art
In a lithography process using an immersion lithography device of high NA (numerical aperture) and an ArF light as an exposure light for manufacturing a semiconductor device of a 45 nm node, the pattern size on a mask is almost the same as the wavelength of the ArF light. In this case, a waveguide effect caused by mask topography and a shadowing effect caused by oblique incident light are not negligible.
In the lithography simulation for designing an LSI, the thickness of a mask and incident angle of an exposure light beam to the mask must be taken into careful consideration when simulating an intensity distribution of the irradiated light on the substrate surface through the mask. Namely, a strict calculation for obtaining a solution of Maxwell's equation is essential, considering the three-dimensional structure of the mask. This calculation, however, takes 10 to 100 times longer than the thin mask approximate calculation in a simulation that does not consider the thickness of a mask. In the actual development, design speed is an important factor and this increase in the calculation time becomes a serious problem.
For the purpose of avoiding the problem, a method (for example, Jpn. Pat. Appln. KOKAI Publication No. 2007-273560) has been proposed in which a three-dimensional mask pattern is redefined as a flat type mask pattern (thin mask pattern), taking into consideration geometrically the size of a light-shielding portion (including a semitransparent portion) and the size of a transparent portion when the three-dimensional mask pattern is irradiated with the exposure light by a light source, hence to obtain an optical image substantially equal to that obtained through a strict calculation taking the incident angle of the exposure light into consideration, in a short time. In this method, however, a phase difference of the light-shielding portion in the thin mask approximate calculation is constant regardless of the incident angle of the exposure light and therefore there is a fear that the accuracy of detecting out-of-focus is low.
As mentioned above, according to the light intensity distribution simulation method of measuring the intensity distribution of the irradiated light on a substrate surface through a mask, accurate measurement is possible but an enormous calculation time is necessary when carrying out the solution calculation of the Maxwell's equation taking the three-dimensional mask structure into consideration. On the other hand, when carrying out the thin mask approximate calculation with the three-dimensional mask pattern redefined as the flat type mask pattern as shown in Jpn. Pat. Appln. KOKAI Publication No. 2007-273560, it is possible to reduce the calculation time but an accurate measurement is not possible due to the adverse influence of the light-shielding portion.